The present invention relates to chroma gain control for color television receivers wherein the chroma gain setting is automatically obtained by use of a vertical interval reference (VIR) signal presently received on line 19 of each field of an NTSC video waveform.
Video information transmitted to a television receiver comprises a luminance signal portion and a chrominance signal portion. The chrominance signal portion comprises color difference signals which are obtained by demodulation of the chrominance signal. The color difference signals are in turn applied to a matrix amplifier where they are combined with the luminance signal to produce red, green and blue color signals for application to the color picture tube.
The ratio of the chrominance signal, from which the red, green and blue color signals to the picture tube are derived, to the luminance signal, used to derive these color signals, is known as the chrominance to luminance matrix ratio and this ratio determines the saturation or chroma gain of the receiver. As the ratio is increased, the resulting picture is more saturated and more intense with color and as the ratio is decreased, the resulting picture is less saturated and less intense with color.
Without the use of a VIR signal, the chrominance to luminance matrix ratio is typically selected by viewer adjustment of the gain of the chrominance signal amplifier in the color television receiver until the resulting picture reaches a visually pleasing degree of color saturation. However, with a VIR signal, the chrominance to luminance matrix ratio is set in accordance with the VIR signal. For example, in U.S. Pat. No. 3,950,780, by Harry T. Freestone, and assigned to the assignee of the present application, the gain setting of a television receiver chrominance signal amplifier is continually updated by a closed loop control circuit using VIR signal information.
In the Freestone patent, the control circuit compares a blue color signal derived from the chrominance reference portion of the VIR signal with a reference signal and adjusts the gain of the chrominance signal amplifier until the two signals are identical. The control circuit thereby insures that the saturation or chroma gain of the receiver is automatically set to the value established by the VIR signal regardless of variations due to aging of the receiver circuitry for processing the chrominance signal. Since the chrominance signal is adjusted in accordance with the VIR signal and since the VIR signal is subjected to the same program deviations experienced by the chrominance signal as transmitted, the VIR signal adjusts the receiver to compensate for such deviations.
A limitation in prior art VIR control circuits is that there is no provision by which a viewer can deviate the chrominance to luminance matrix ratio from that automatically set by the VIR signal. Some viewers may, for example, prefer greater color saturation than that provided by this chrominance to luminance matrix ratio. Other viewers may prefer less color saturation. The present invention envisions the utilization of a closed loop control circuit which allows for viewer selection of a preferred deviation from the color saturation determined by a VIR signal.
One method by which the preferred viewer deviation of color saturation might be considered to be achievable is to include an offset voltage to a closed loop control circuit, such as that taught by the Freestone patent, to adjust the gain of the chrominance signal amplifier a fixed amount from the setting determined under the control of the VIR signal. Unfortunately, such interjection of an offset voltage fails to maintain a preferred chrominance to luminance matrix ratio when luminance amplification is altered by adjustment of the receiver contrast control.
Another disadvantage of attempting to realize chroma gain preference control by means of an offset voltage in a control loop including the RGB matrix is that the blue color signal used in such prior art control loops for adjustment of chrominance signal amplifier gain is obtained from the blue drive of the color television receiver picture tube. This blue drive is a high level signal which contains high frequency components and thus poses radiation control problems. In addition, this blue drive in some cases is derived by matrixing of the luminance signal with color difference signals after the luminance signal is subjected to vertical blanking. As a consequence, erratic tolerance of receiver components creates a possibility that the blue drive will be lost during VIR signal transmission due to erroneous vertical blanking of line 19.
It is accordingly an object of the present invention to add viewer preference chroma gain deviation capability to a color television receiver wherein chroma gain is VIR controlled.
Another object of the present invention is to add viewer preferred chroma gain deviation to such a color television receiver which is independent of contrast control adjustment.
Still another object of the present invention is to add viewer preferred chroma gain without use of a high level blue drive and without risk of malfunction due to unintentional vertical blanking during line 19.